1. Industrial Field of the Invention
The present invention relates to a fuel vapor processing apparatus of an internal combustion engine in a vehicle.
2. Description of Relative Art
In a conventional fuel vapor processing apparatus, fuel vapor generated in a fuel tank while an engine of a vehicle is stopped is absorbed by absorbents in canisters, and the absorbed fuel vapor is purged while the engine is operated, so that it is supplied to the engine through a suction air passage and burned in the engine. In such a known apparatus, as disclosed in Japanese Utility Model Unexamined Publication No. 61-25568, a main canister and a sub-canister are connected in series, and also, the sub-canister is provided closer to the atmospheric side than the main canister is.
In this apparatus, the sub-canister is located on the atmospheric side of the main canister because such an arrangement suppresses release of the fuel vapor into the atmosphere when the pressure in the fuel tank is increased or decreased and the fuel vapor is not adequately absorbed in the main canister.
More specifically, during driving of the vehicle, the fuel vapor in the canisters is completely purged. Then, when the engine is stopped, the temperature in the fuel tank falls during the night, and the pressure in the fuel tank becomes negative to the atmospheric pressure. In such a case, the air is introduced from the outside (the atmosphere) via the sub-canister and the main canister in order to prevent breakage of the tank.
Next, in the daytime, when the temperature in the fuel tank rises, fuel vapor is generated, and the pressure in the fuel tank becomes positive to the atmospheric pressure. Then, the vapor and the air in the fuel tank is released into the atmosphere via the main canister and the sub-canister in order to prevent breakage of the tank.
In this case, most of the fuel vapor is absorbed in the main canister, and an amount of the fuel vapor which is absorbed in the sub-canister connected to the atmospheric side of the main canister is small, so that it is possible to reduce an amount of the fuel vapor which is released into the atmosphere.
In the conventional technique described above, if the engine is stopped and left as it is for a long time, the fuel vapor in the main canister diffuses and flows into the sub-canister, and consequently, the concentration of the fuel vapor in the sub-canister reaches the same value as that of the fuel vapor in the main canister.
In this state, when the temperature in the fuel tank rises and the fuel vapor in the fuel tank is released into the atmosphere via the main canister and the sub-canister, the fuel vapor having the same concentration as the fuel vapor in the main canister is released from the sub-canister into the atmosphere, thereby polluting the atmospheric air considerably.
In order to solve such a problem, the area of a passage which communicates the main canister with the sub-canister is decreased to reduce diffusion of the fuel vapor into the sub-canister. However, it results in a new problem that the purge flow rate during operation of the engine is decreased, thereby deteriorating the purge performance.